Past lives caught in the dust of trees

I’m cur­rently work­ing at the Annals of Botany to help out with their social media side. There’s a bit more to it than subtly drop­ping links to their site, like this one. At the moment I’m strug­gling with the Facebook integ­ra­tion, but there’s a fun side too. I wouldn’t have browsed AoB if I’d not been hired, and that means I would have missed out on papers like Phytoliths in woody plants from the Miombo wood­lands of Mozambique by Julio Mercader and his team at Calgary. I’ll admit the art­icle title doesn’t say much to the lay­man, but it’s actu­ally some­thing deeply cool that I didn’t find out about till my MPhil.

If mega­liths are big stones and micro­liths are small stones like arrow­heads, then phyto­liths are clearly phyto-stones. Phyto– in this case mean­ing plant.

Phytoliths are micro­scopic stones formed in some plants. When a plant’s roots draw up water they also draw up the min­er­als dis­solved within it. In the case of the silica this gets pulled out of the water and depos­ited either in the cells or between the cells. The exact shape of the phyto­liths var­ies on the part of the plant the silica is depos­ited in, the avail­ab­il­ity of silica and, most excit­ingly for archae­olo­gists, the spe­cies of the plant.

Phytoliths are use­ful because nor­mally bio­lo­gical mater­ial doesn’t hang around long in the soil. Once some­thing is dead it’s lunch for some­thing else. Phytoliths are bio­lo­gical mater­ial but not organic, so they don’t break down in the same way. Mercader et al. are clear that’s is not an unam­bigu­ous relal­tion­ship. Time still has an effect, but it’s easier to find phyto­liths than it is to find organic remains for plants. Still as use­ful as they are, phyto­liths alone are not enough. A hand­ful of phyto­liths under a micro­scope would just look like a nobbly (or smoothish) thing. If you haven’t seen what a baobab phyto­lith looks like, you’re not likely to guess from simply look­ing at the phyto­lith and this is where Mercader et al step in.

The area stud­ied was a tran­sect through the forest between the Lake Niassa shore at Metangula and the high­lands at Njawala, a dis­tance of 50km and a rise from 465m above sea-level to 1841 above sea-level. They also used indi­gen­ous col­lect­ors to sample the flora in a 5km radius around Metangula and Njawala. They estim­ate they got over 90% of the spe­cies used by the nat­ive peoples. Given that a lot of usage is likely to be dom­in­ated by rel­at­ively few spe­cies, that’s a lot of plant mater­ial. There’s then a LOT of list­ing and descrip­tion of phytoliths.

The com­mon fea­ture that amazes me is how small many of these phyto­liths are. Some are just 20–40 μm long. A micro­metre (μm) is one thou­sandth of a mil­li­metre. Despite this Mercader et al, point to the phyto­liths at the other end of the scale, some are around 200μm in length and over half are over 50μm. This means if you use stand­ard tech­niques to sieve for phyto­liths using a 50.238 to 63.246μm cut-off, you’ll miss all these lar­ger phyto­liths. That’s going to mat­ter if what you want to find evid­ence of a ‘Zambezian’ forest at an archae­olo­gical site.

It’s the sort of sci­ence that is easy to over­look. The authors don’t con­clude that whole text­books need to be re-written or that our under­stand­ing of Africa’s past has to be rebuilt from scratch. It’s also the kind of sci­ence that’s easy to whine about. Here they are, pick­ing flowers to exam­ine tiny stones in the stems rather than just appre­ci­at­ing the beauty.

But it’s also the kind of sci­ence that increases the amount of beauty and mys­tery in the world.

Until I took my MPhil, I was com­pletely ignor­ant of phyto­liths. I could view the same plants an archaeo­bot­an­ist, but saw a lot less. Before I read this paper I didn’t know that that the Miobos wood­lands were unex­amined. Knowing that these things are out there opens up new pos­sib­il­it­ies for what can be done. At Çatal­höyük they’re examin­ing phyto­liths left behind in what are almost shad­ows of woven bas­kets to flesh out details of human life in the past. In the case of this paper, it provides a bench­mark for meas­ur­ing future study­ing against. It’s detailed, metic­u­lous and some­times opaque to the non-specialist, but it’s also a descrip­tion with last­ing value. Currently pub­lic­a­tions are often judged on cita­tions garnered over a few years. That misses the value of this paper as it will be import­ant for dec­ades. Indeed, if this eco­sys­tem sud­denly becomes a tar­get for eco­nomic devel­op­ment it could even be import­ant for cen­tur­ies as a snap­shot of the cur­rent state of the Miombos woodlands.

If you want to see the phyto­liths they found, you can down­load the paper for free.

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When he's not tired, ill or caught in train delays, Alun Salt works part-time for the Annals of Botany weblog. His PhD was in ancient science at the University of Leicester, but he doesn't know Richard III.

1 Response

This is a per­fect example of why social net­work­ing is going to be so use­ful for sci­ence, and why I, as editor, am so keen that Annals of Botany is involved! We pub­lish a lot of inter­dis­cip­lin­ary papers and the new tag­ging and dis­tri­bu­tion will let people know about them. We even pub­lish some spec­tac­u­lar art his­tory — the medi­eval manu­scripts known as the Tacuinum Sanitatis show how cucum­bers and eggplants/aubergines looked when first grown, and the paper by Paris, Jaunay and Janick has some spec­tac­u­lar plates — http://​dx​.doi​.org/​1​0​.​1​0​9​3​/​a​o​b​/​m​c​p​055 — even show­ing the ways man­drake roots were har­ves­ted! Fortunately, I don’t grow this in my veg.patch any more, since the scream when you pull up the roots is shown to be fatal.